Currently, the babel package does not provide a Japanese option.
The file japanese.dtx defines all the language definition macros
for the Japanese language.
Note that this package works with a latex system *pLaTeX2e* and
documentclass such as jarticle, jbook, jsarticle and jsbook.
1) Process japanese.ins with platex.
2) Process documentation, japanese.dtx and sample.tex, as you wish.
3) Deploy the directory or files in an appropriate place under
the TeX tree.
4) Execute mktexlsr.
Copyright (C) 1999-2007 by ING
This package is free software that can be redistributed and/or
modified under the terms of the LaTeX Project Public License as
specified in the file `macros/latex/base/lppl.txt on any CTAN
archive server, such as http://www.ring.gr.jp/archives/text/CTAN/.
Please report errors to: ING